This invention relates to a nematic liquid crystal composition. More particularly, the invention relates to a nematic liquid crystal composition, the direction of the molecular axis of which is changed without occurrence of dynamic scattering by application of an electric field, a magnetic field and the like.
It is known that nematic liquid crystals (hereinafter referred to as "N-liquid crystals") can be used for display, light modulation and the like by utilizing their specific property that optical characteristics are changed under application of electric fields, magnetic fields, ultrasonic waves and the like. Such elements comprise, in general, an N-liquid crystal filled and supported between two substrates disposed to confront each other with a distance smaller than about 50 .mu., at least one of which is transparent, and the change in the molecular arrangement caused under application of electric fields, magnetic fields, ultrasonic waves and the like is utilized for light modulation.
Compounds forming such N-liquid crystal are divided into two types depending on the molecular structure and dielectric properties, one type being characterized in that the molecular axis and electric dipole are substantially vertical to each other (the liquid crystal of this type will hereinafter be referred to as "Nn-liquid crystal") and the other being characterized in that the molecular axis and electric dipole are substantially in parallel (the liquid crystal of this type will hereinafter be referred to as "Np-liquid crystal"). Accordingly, the Nn-liquid crystal indicates an N-liquid crystal having a negative dielectric anisotropy, and the Np-liquid crystal indicates an N-liquid crystal having a positive dielectric anisotropy.
In N-liquid cyrstal electro-optical elements heretofore generally used, the mechanism of scattering caused by application of an electric field or the like, which is known as "dynamic scattering mode (DSM)," is utilized, and Nn-liquid crystals are used for such elements. Most of known N-liquid crystals belong to the Nn-type.
It is also known that elements comprising an Np-liquid crystal filled and supported between substrates can be utilized for light modulation. Such elements are prepared by rubbing surfaces of substrates in one direction with fabric, paper or the like, piling two rubbed substrates so that the rubbing directions in the substrates are in parallel or cross each other with a certain angle, and injecting a liquid crystal between the so piled substrates. Between the substrates, the molecular axes of the Np-liquid crystal are made in parallel with the substrate faces and are arranged in the substantially same direction in a plane parallel to the substrate. In case, for example, the two substrates are disposed so that the rubbing directions cross each other with a certain angle, if seen from the direction vertical to the substrates, molecular axes are arranged in the state continuously distorted from one another between adjacent faces, and on the interface between the liquid crystal and substrate the molecular axes are aligned in the same direction as the rubbing direction. When polarized light passes through this liquid crystal layer, the polarization plane of the light is rotated depending on the degree of distortion of the molecular axis direction. This distortion of the molecular axis direction can be relaxed by application of an appropriate electric or magnetic field. Accordingly, by adjusting the intensity of the electric or magnetic field it is made possible to adjust rotation of the polarization plane of polarized light passing through the element.
Since the light modulation process utilizing an Np-liquid crystal electro-optical element formed between two polarizing plates disposed to cross each other exhibits a mechanism quite different from an Nn-liquid crystal electro-optical element, a higher contrast ratio can be obtained when used for display of a pattern. Further, the Np-liquid electro-optical element can be utilized for construction of a Boolean algebra generator, a logical product gate, a "nor" gate and a more complicated logical circuit. If an Np-liquid crystal electro-optical element capable of responding quickly is employed, it can be utilized for display of a three-dimensional television or moving picture.
The threshold value causing DSM in an Nn-liquid crystal electro-optical element is about 7 to about 10 V.sub.RMS and the saturation voltage is about 40 V.sub.RMS, and the Nn-liquid crystal electro-optical element is generally actuated under about 25 to about 40 V.sub.RMS. In contrast, in the case of an Np-liquid crystal electro-optical element, the threshold value is about 1.5 to 4 V.sub.RMS and the saturation voltage is about 7 to about 10 V.sub.RMS. Accordingly, a lower voltage electric source can be used, and the consumption of electric power can be reduced and the life of the element can be prolonged. Further, in the case of an Np-liquid crystal electro-optical element, since the wavelength region of rays of transmission varies depending on the electric voltage when it changes within the range of the threshold value to the saturation voltage, it can be used for display of colors. Moreover, also a device comprising an Np-liquid crystal element filled and supported between electrodes in which the direction of rubbing electrode faces are arranged in parallel can be used for display of colors.
In case an Np-liquid crystal electro-optical element is so constructed that the molecular axis direction are random between substrates, the light is scattered when no electric field is applied to the element, and when an electric field is applied, the quantity of transmission light increases because molecular axes are oriented vertically to the substrates. In this case, use of a polarizing plate is unnecessary.
Most of N-liquid crystals that are presently known in the art are Nn-liquid crystals, and N-liquid crystal substances and compositions which take a nematic mesophase at least at room temperature and have a positive dielectric anisotropy are hardly known in the art.
In Japanese Patent Application KOKAI No. 18783/72, it is disclosed that a mixture of an Nn-liquid crystal and 4-cyano-benzylidene-4'-n-alkylaniline is a liquid crystal composition having the same electro-optical characteristics as those of an Np-liquid crystal.